1. Field of the Invention
The present invention relates to a display device using an n-channel thin film transistor and a p-channel thin film transistor.
2. Description of the Related Art
In recent years, a technology to form thin film transistors using semiconductor thin films (with a thickness of approximately several tens to several hundreds of nanometers) that are formed over substrates having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices and rapidly developed especially as switching elements for image display devices.
A Thin film transistor using an amorphous silicon film for a channel formation region, a thin film transistor using a polycrystalline silicon film for a channel formation region, or the like are used as a switching element in an image display device. As a method for forming polycrystalline silicon films, a technology is known in which a pulsed excimer laser beam is processed into a linear shape by an optical system and an amorphous silicon film is scanned and irradiated with the linear beam for crystallization.
In addition, as switching elements of image display devices, thin film transistors using a microcrystalline silicon film as a channel formation region are used (Patent Document 1: Japanese Published Patent Application No. H4-242724, Non-Patent Document 1: Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373).
A thin film transistor using a polycrystalline silicon film for a channel formation region has an advantage in that mobility is higher than that of a thin film transistor using an amorphous silicon film for a channel formation region by two or more digits, and a pixel portion and a peripheral driver circuit of a semiconductor display device can be formed over the same substrate. However, the thin film transistor using a polycrystalline silicon film for a channel formation region requires a more complicated process than the thin film transistor using an amorphous silicon film for a channel formation region because crystallization of the semiconductor film is required. Therefore, there are problems such as reduction in yield and increase in cost. Further, the sizes of crystal grains of a polycrystalline silicon film formed by irradiating an amorphous silicon film with an excimer laser beam readily vary because energy of the laser beam varies. Therefore, electric characteristics of a thin film transistor vary when it is formed using such a polycrystalline silicon.
On the other hand, the manufacturing process of a thin film transistor using an amorphous silicon film for a channel formation region is simple, since it can be formed with a few photomasks. However, fluctuation of the threshold of the thin film transistor is large and the mobility of the thin film transistor is low. Moreover, it is difficult to manufacture a p-channel thin film transistor when an amorphous silicon film is employed for a channel formation region.
In the case of an inverted-staggered thin film transistor using a microcrystalline silicon film for a channel formation region, crystallinity in an interface region of a gate insulating film and a microcrystalline semiconductor film is low and electric characteristics of the thin film transistor is poor. Additionally, it is difficult to manufacture an inverted-staggered p-channel thin film transistor when a microcrystalline silicon film is used for a channel formation region.